From movement toward plastic material integration in various industries in recent years, particularly a polypropylene resin is expanding industrial fields because of its excellence in the balance among mechanical strength, heat resistance, processability and price and because of its excellent properties in easy combustibility and easy recyclability. Similarly, polypropylene resin foamed bead molded articles obtained by molding of polypropylene resin foamed beads can be endowed with characteristics such as lightweight properties, shock-absorbing characteristics and thermal insulating properties without losing excellent properties of the polypropylene resin and are thus widely used as packaging materials, building materials, insulator materials, automotive materials etc.
A polypropylene resin foamed bead molded article obtained by molding polypropylene resin foamed beads is superior to a polystyrene resin foamed bead molded article in heat resistance, chemical resistance, rigidity, and compression strain recovery. On the other hand, when polypropylene resin foamed beads are secondarily foamed and simultaneously fused in molding, high-temperature heating as compared with molding of polystyrene resin foamed beads, that is, heating by steam at high saturation vapor pressure, is necessary. Accordingly, a mold with high pressure-proof specifications and a dedicated molding machine for high press pressure are necessary, and energy costs used therefor is also high.
To solve this problem, a method of coating the surfaces of polypropylene resin foamed beads with low-melting different resins is also carried out (for example, Patent Document 1), but the apparatus used in this coating is complicated and the production process thereof is also complicated, and the mutual fusion of the resin particles in molding is improved, but because the secondary foaming of the foamed beads is not sufficient, there remains room for improvement in the external appearance of the molded article, and for the purpose of improving the secondary foaming property of foamed beads for solving this problem, there is necessity for a step of giving internal pressure for significantly increasing the internal pressure of foamed beads, for molding by in-mold filling at high compression ratio, or for increase of steam pressure during a molding opposite to desired objects.
There is also a method of using a relatively low-melting polypropylene resin as a base resin, and polypropylene resin foamed beads using, as a base resin, a polypropylene resin polymerized by using a metallocene polymerization catalyst are known (for example, Patent Document 2). The polypropylene resin polymerized using the metallocene polymerization catalyst gives a low-melting resin more easily than with a polypropylene resin polymerized with the conventional Ziegler-Natta catalyst, but there is still room for improvement in the effect of reducing the saturation vapor pressure of steam necessary as a heating medium during a molding, or for maintaining excellent properties such as mechanical strength and heat resistance inherent in the polypropylene resin.
Non-crosslinked foamed beads using a modified polypropylene resin as a base resin having [content of a polypropylene resin]/[content of a polymer composed of a vinyl monomer] at a ratio of 97 to 65% by weight/3 to 35% by weight, obtained by graft-polymerizing a polypropylene resin impregnated with a vinyl monomer are known (for example, Patent Document 3). The heat resistance performance of foamed beads using the modified polypropylene resin particles as the base resin tends to depend on state transition unique to the high-molecular polymer such as melting point or glass transition temperature, and by selecting a polypropylene resin having a low melting point, the saturation vapor pressure of steam during a molding can be reduced, but there remains a problem in maintenance of heat resistance.    Patent Document 1: JP-A 2000-894    Patent Document 2: JP-A 6-240041    Patent Document 3: JP-A 10-292064